Because business threads are labeled as such, so they move on to itchy nuts, cowbell, hit this, and so on.
But also they are generally well written, longer than a sentence, with correct punctuation. Meaning, you are going to get a lot of pictures of how it's too long to read.
But then some others will actually take the time to write out a well thought out reply. Just to have some picture of Colin Powell saying, "I ain't going to read dat shit", or something.
If it's longer than a few sentences, missing a punchline, or has a purpose. Then it's on page 20 within an hour.
Oh, and don't forget all of the.. sig spot secured, siggy niggie and so on.